During the manufacture of machine parts and metallic components, including but not limited to engine power transmission and geared elements, testing of the constituent parts, or representative samples there from, is an essential method for quality control, and this is of particular importance in certain fields, such as vehicles, turbine, aviation and aerospace applications, in which component failure could have catastrophic results.
For example, aircraft engines are quite different from most automobile engines as they rely entirely on available air-cooling to control engine oil and part temperatures. The engine is exposed to significant changes in temperature as a result of changing airflow and rapid expansion and cooling of the engine parts occur. This can cause premature engine parts failure, reduction of power and oil leaks to occur.
On most automotive engines manufactured today, the engine temperature is cooled by water circulation and is automatically temperature controlled using a thermostat thus eliminating the vast engine temperature changes that occur in air-cooled aircraft engines.
Because aircraft engines are subjected to this rapid expansion and contraction of engine parts any residual stress that is in the engine parts can significantly affect the life of an engine. Residual stress occurs in parts at rest, and may be a by-product of manufacturing processes and cyclic use.
The need for such quality testing in which metallic component failure could have a catastrophic result is often not well or completely met in the metallic context by presently available testing methods. First, destructive testing methods are limited because of the various means in which such part destruction results in distortions which mask or obliterate the forensic value of the particular component under such examination.
Currently there are many forms of Non Destructive Testing Processes (NDT) for metallic materials that include but are not limited to examples as follows: X-ray diffraction (xrd), Radiography (rt), Convergent beam electron diffraction (cbed), Transmission electron microscopy (tem), Neutron diffraction, Synchrotron hard x-ray, Eddy current (et), Magnetoelastic instrumentation barkhausen noise (bn), Ultrasonic resonant analysis, Magnetoacoustic, Ultrasonic, Thermoelastic infared, Photoelastic, Electronic speckle pattern interferometry, Magnetic particle, Magnaflux quasar process compensated resonant inspection (peri), and Acoustic resonance. Other forms of non destructive testing have limitations in their practical value due to, for one example, refraction issues in the x-ray context.
The present subject matter may augment, incorporate, modify or substitute for the above identified testing procedures.
Of particular importance is the detection of residual stress in a part or product. Residual stress is stress present in a body that is free of external forces or temperature gradients. Residual stress can be induced through manufacturing processes such as heat treating, machining, shot peening, forming, grinding, casting and other procedures that have been applied to a material.
Under typical parts, manufacturing conditions, temperature gradients can produce non-uniform dimensional and volume changes. When metal castings cool and solidify, compressive stresses develop in lower-volume areas, which cool first, and tensile stresses develop in areas of greater volume, which are last to cool. Shear stresses can develop between the different volume areas. This can happen even in large castings and machine parts of relatively uniform thickness. The surface cools first and the core last. In such cases, stresses develop as a result of the phase (volume) change between those layers that transform first and the center portion, which transforms last.
When both volume and phase changes occur in metal parts of uneven cross section, normal contractions due to cooling are opposed by transformation expansion. The resulting residual stresses will remain until a means of relief is applied. This type of stress develops most frequently in steels during a quenching process frequently used in parts manufacturing. As a result, the surface becomes harder before the interior does. Although the inner materials can be strained to match this surface change, subsequent interior expansions place the surface of the metal under tension when the inner material transforms. Cracks in high-carbon steels can arise from such stresses and cause pre-mature parts failure when under load stress.
Grinding operations, when parts are machined, may cause residual stresses in parts such as crankshafts, camshafts and gears. During an initial grinding process the part being ground will have an elevated surface temperature as a result of the grinding wheel contact. The surface of the part being ground becomes heated while the surrounding metal constrains expansion around the grinding area. As the machined metal cools after grinding it can leave a tensile residual stress on the surface. At a later point in time as the part is subjected to operational stresses from normal engine operation, surface cracks can develop causing premature parts failure.
One example of destructive testing for residual stress is presented in “Cross-Sectional Mapping of Residual Stresses by Measuring the Surface Contour After a Cut” M. B. Prime, Journal of Engineering Materials and Technology. Volume 123, April 2001, pp. 162-168, the entirety of which is incorporated herein by reference. In Prime and other methods, residual stresses may be determined from deformation measured after material is removed and destroyed. These methods have the disadvantages of destructive testing as described previously.
One common mechanism, in addition to manufacturing induced stress, which introduces residual stress is the transformation or incomplete transformation of austenite to martensite in the manufacture of steel parts.